On which walls can these suction cups stick to?
They are suitable for tiles, artificial stones, glass, metal, not for painted wall, rough surface and wallpaper.
Is there some sort of trick to get suction cups to stick?
Make sure that the sink surface and suction cups are clean, then a slight spray of water on the suction cups will make them adhere more firmly to the surface, in addition to this, make sure to press on the suction cups to remove air.
What they are made of ?
Made from 304 stainless steel, they are robust and will not rust even after prolonged exposure to moist environment.